A. Field of the Invention
The present invention relates generally to methods and apparatus for cutting or perforating objects and more particularly to apparatus and methods for chemically cutting objects within a well bore by making multiple cuts with fluid jets.
B. Description of the Prior Art
Chemical cutting devices or tools are well-known within the art and are frequently used to cut, sever, perforate or slot objects within a well bore. Such objects may include metal pipe, well bore casing, earth formations, or foreign objects such as lost tools which may be found within the well bore. Chemical cutting is advantageous in downhole situations because it does not produce debris that must be removed from the hole and it does not flare the cut end of the pipe.
Such known devices are typically tubular structures which enclose a chamber containing a cutting fluid that is extremely active chemically and which reacts violently when it is brought into contact with most oxidizing substances. Examples of such cutting fluids include fluorine, and halogen fluorides including such compounds as chlorine trifluoride, chlorine monofluoride, bromine trifluoride, bromine pentafluoride, iodine pentafluoride and iodine hexafluoride and mixtures thereof.
Generally, chemical cutting fluids are discharged in one or more high velocity streams or jets by applying to the chamber in which they are stored a suitable pressurizing agent. Pressurizing agents generally include hydraulic or pneumatic fluids. Pneumatic fluids may be gases generated by the ignition of one of the various types of relatively slow-burning gun powders or other explosives including black powder, rocket propellant powders, or the like. By appropriate selection of an explosive and by means of preparation procedures well-known to those skilled in the art, the ignition and burning rates of such explosives may be effectively controlled to generate gases at any desired rate and volume suitable for applying the desired pressurizing forces to the cutting fluid.
Typically, the chemical cutting is accomplished in a single "one shot" operation. If it is desired to completely sever a conduit within a well bore, a cutting tool having multiple discharge nozzles adapted to discharge radial jets of cutting fluid in all directions is disposed within the conduit. When the propellant is fired, the cutting tool is anchored in the conduit and the cutting fluid discharged through the nozzles severs the conduit. There are, however, certain limitations on the effectiveness of such tools. For example, the standoff distance for such tools is relatively short. Standoff is defined as one-half the difference in conduit internal diameter and cutting head outside diameter. Thus, the standoff is the radial distance between the cutting head and the inside of the conduit when the cutting head is centered in the conduit. The maximum effective standoff distance in chemical cutting tools is on the order of one-half inch or less. Thus, the outside diameter of the cutting head is optimally less than one inch smaller than the inside diameter of the conduit to be cut. The standoff constraint thus limits the ability of the tool to go through diameter restrictions and cut larger diameter conduits.
Additionally, there is a limit to the thickness of conduit that may be cut. During the cutting, substantial heat is generated. As the amount of cutting fluid is increased in order to cut thicker conduit, the amount of heat generated by the cutting is increased. During attempts to cut extremely thick conduit, the heat generated may become so great as to destroy the cutting tool itself.